What is a Plastid?

Introduction

In eukaryotic cells, photosynthesis occurs mostly within the organelles known as plastids. Plastids include chloroplasts and any other pigment-containing cytoplasmic organelles that allow the plant to use light and carbon dioxide to synthesize food and energy. Plastids, double-membrane organelles, which are mostly found in eukaryotic cells, may be divided into two categories based on their membrane structure −

• Primary Plastids.

• Secondary Plastids.

Most algae and plants include primary plastids, whereas plankton, such as diatoms and dinoflagellates, generally possess secondary or more complex plastids.

It's important to study the genesis (origin) of plastids since it helps us comprehend photosynthesis in green plants which is our main food supply.

Origin of Plastid

• Plastids are semi-autonomous cells with their own genetic material and ribosome translating machinery. Like mitochondria, plastids are inherited from parents. Proplastids replicate in daughter cells during plant structure development. Plastids are formed from pre-existing plastids because of heredity. Inheritance of chloroplasts is a nurturing, non-Mendelian cytoplasmic type. The nuclear genome controls Mendelian inheritance, whereas the plastogenome controls cytoplasmic inheritance.

• Plastids have their own genome, but chloroplast growth requires nuclear gene coordination and its products. For example, Euglena separates proplastids without cell division, but continued development requires nuclear gene and plastogene products.

• When proplastids are exposed to light, they slowly change their colour to green and grow big. This is accompanied by the growth of granal assemblies. The inner chloroplast membrane forms finger-shaped vesicles throughout the time. They pinch off many membrane vesicles, which are assembled at the centre. The vesicles start to join together, and in the end, they form clusters of thylakoid membranes called Grana. Inner membranes disintegrate after chloroplast formation.

• According to the endosymbiont theory, most of the endosymbionts' genes were transported to the nucleus before they were completed. New mitochondrial genetic code improvements affect the remaining mitochondrial genes malfunctioning if carried to the nucleus, preventing further transfers.

Structure of a Plastid

Plastids are surrounded by two-unit membranes with an outer membrane an inner membrane of 7 nm thick. These membranes are separated by a periplastid space 8-10 nm thick. The inner membrane of fully formed plastids does not fold inward like mitochondria, but it is active in the evolution of proplastids into mature ones.

Stroma, containing grana, fills chloroplast. The stromatic fluid comprises grana, enzymes, plastid DNA, RNAs, and 70s ribosomes. Photosynthesis-related plastids have several membrane layers.

Plant nuclear genes convert most plastid proteins, and their expression is extensively co-regulated to regulate plastid development with cell differentiation. Plastid nucleoids are huge protein-DNA complexes attached to the internal envelope membrane.

Types of Plastids

There are different types of plastids based on the presence or absence of the pigments and their stages of development. The chief ones are as follows −

• Chloroplasts.

• Chromoplasts.

• Gerontoplasts.

• Leucoplasts.

• Proplastids

Chloroplasts

Chloroplasts are biconvex, semi-porous, double-membraned organelles that are located within the mesophyll of plant cells. They produce food through photosynthesis. It possesses the photosynthetic pigment chlorophyll which is responsible for its characteristic green colour and for capturing sunlight which gets converted into useful energy, thereby, releasing oxygen and water molecule.

Chromoplasts

All plastids that contain several colourful pigments are classified as chromoplasts. Typically, they are present in blossoming plants, ageing leaves, and fruits. Chloroplasts are transformed into chromoplasts. Chromoplasts contain carotenoid pigments, which give plants and fruits their various colours. The primary purpose of its unique colour is to attract pollinators.

Depending on the predominant pigments found in plastids, they are further subdivided into various forms. For example, Protoplasts are enriched with red pigment, phycoerythrin. Yellow pigments, such as xanthophylls and carotenoids, are present in phenoplasts and xanthoplasts. In addition to the pigments listed above, phycocyanin and other pigments are found in different coloured plastids.

Gerontoplasts

Gerontoplasts are the chloroplasts of leaves that transform into many other organelles when photosynthesis is no longer taking place, frequently in the autumn. These typically accompany the ageing process.

Leucoplasts

Leucoplasts are non-pigmented plastids that are usually present in storage parenchyma and other non-pigmented tissues. These are found in the non-photosynthetic plant parts, such as roots. Most of them serve as a storage facility for carbohydrates, lipids, and proteins, according to the plant's demands.

Their principal purpose is the conversion of amino acids and fatty acids. If such leucoplasts are exposed to sunlight, they will convert into colourful plastids, suggesting that these plastids have all the genetic capability to develop and perform photosynthesis.

Leucoplasts are further categorised according to the chemicals they store, they are -

• Amyloplasts – Amyloplasts are the largest leucoplasts of the three and are responsible for storing and synthesising starch.

• Proteinoplasts – Proteinoplasts, also known as aleuroplasts, are commonly present in seeds and help plants store the proteins they require. They serve a critical part in the storage of food and for the metabolism of energy in plant cells, which is their primary function.

• Elaioplasts - Elaioplast aids in reserving fats and oils required by the plants. They are largely located in the layer of cells surrounding budding pollen grains in the anther.

Proplastids

Meristematic cells include tiny, transparent, juvenile vesicular structures that lack pigmentation known as proplastids. Depending on the organ and the amount of light, proplastids turn into colourless leucoplasts, brightly coloured chromoplasts, or green chloroplasts. Proplastids divide repeatedly to make cells that can change into different types.

Functions of Plastids

• Plastids synthesize fatty acids and terpenes, which may be utilised to generate energy and synthesise other compounds. For example, palmitic acid, generated in mesophyll tissue chloroplasts, is used by epidermal cells to make plant cuticle and epicuticular wax.

• Proplastids in plant meristematic areas give rise to all plastids. Mature chloroplasts can split by binary fission like proplastids and juvenile ones.

• Plastids are another plant-only energy-transducing cell organelle. Schimper coined the term Plastids for photosynthetic structures. Photosynthesis delivers chemical energy directly or indirectly, but only chloroplasts can capture, transform, and store solar energy as chemical energy.

• Plastids produce and store essential chemicals for autotrophic eukaryotic cells. Plastids include photosynthetic pigments that define the cell's colour. Like prokaryotic organisms, they have circular double-stranded DNA and a common evolutionary origin.

Conclusion

All plant plastids contain the same circular genome and develop from proplastids in immature plant meristem cells (undifferentiated plant tissue). Chloroplasts store energy and synthesise metabolic components. Green chloroplasts absorb light for photosynthesis, whereas yellow-to-red chromoplasts create carotenoids.

The colourless leucoplasts produce carbohydrates, oils, and proteins. Dissolved enzymes make up most of the chloroplast's semi-fluid stroma. Chloroplasts, like mitochondria, have DNA and stroma-specific ribosomes and RNAs. Chloroplasts and mitochondria in plant cells generate energy.

Higher plant stroma has lamellae, internal membranes with stacks (granums) of closed hollow discs called thylakoid connected by lumens connect. Water-absorbing chlorophyll molecules emit electrons. The stroma fixes low-energy carbon dioxide into high-energy glucose.

FAQs

Q1. Who coined the term plastid?

Ans. E. Haeckel discovered and termed plastids, but A. F. W. Schimper was the first to provide a precise description.

Q2. How were plastids inherited?

Ans. Many plants get their plastids from just one parent. Many gymnosperms get their plastids from the male pollen, but angiosperms get theirs from the female gamete. The plastids in algae come from only one parent. The DNA in the plastids seems to come from only one parent. In hybridisation, plastid inheritance appears to be more random.

Q3. Define Desiccoplasts, Phenyloplasts, and Xyloplasts.

Ans. Desiccoplasts are plastids that can change between chloroplasts and proplastids in desiccation-tolerant plants.

Phenyloplasts are colourful plastids that are rich in phenols and differ from chromoplasts in storage and homeostasis.

Xyloplasts, derived from proplastids or amyloplasts, are present in secondary vascular tissues and synthesise precursors for monolignol biosynthesis.